Saturday, 20 April 2013

High Dynamic Range - It's a bit like Marmite, you either love it hate it.

HDR often gets a bad press and to be honest when someone mentions HDR we all think of those over processed, grungy cartoon like images. Well, it doesn't always have to be like that, in fact, a lot of images you see everyday are most likely created using HDR techniques.

So for those of you who don't like HDR, have never tried to create an HDR image or are just plain curious please read on. This is a short and sweet explanation of why HDR can be a good process to use to get the most of your images. (Disclaimer - I'm not an expert. This is just from experience)

Why HDR ? High Dynamic Range is a process used to obtain more information from a scene than the camera is capable of in one shot. Our eyes are much better than the camera lens/sensor at seeing all of the tonal ranges in a scene. The fact that we can increase and decrease our pupils to suit the brightness of a scene means that we can take in a dynamic range of upto 24 stops. However, that amount of stops is only achievable because our pupils constantly change size as we look around a scene. The camera does not have that capability when taking a single image.

If we were to look at a scene with our eyes and kept our pupils at a fixed size the dynamic range reduces to around 10-14 stops. Most current DSLR's can capture 8-11 stops so now there isn't that much of a difference between our eyes and the cameras eye. The challenge is how to capture those 24 stops ? (or enough stops to show off everything in the scene.)

In order for the camera to mimic our pupils we need to capture over and under exposed images as well as the correct (or middle) image. This isn't limited to 3 shots though, some people take upto 9 shots (-4,-3,-2,-1,0,1,2,3,4).

If you take a typical landscape scene were you have land (foreground) at the bottom of the scene and sky (background) at the top it is difficult to expose for both in the one shot. If you expose for the sky your foreground will be too dark and if you expose for the land your sky will be blown out. So why not just use a grad filter or sort it in Photoshop later !

Well, there are a number of ways to address this issue. Some processes make use of much more data than others and this is why some processes work well and others not so well. If you take your landscape shot using a grad filter you'll get the exposure balanced right in camera but then all of the data you get to process will be captured in one single file. If you take a number of shots (HDR) then you have a a lot more data to play with (especially if you are 32bit)

HDR is one method you can use to capture the required range of stops. As mentioned at the start, it doesn't have to be a grungy cartoon image but simply processed enough to pull the required detail out in the right places. It doesn't have to be 9 shots either, 3 will do...and in some cases only 1 !

Ok, so how can I do a HDR image with one shot ? Well technically not HDR but if you take 3 bracketed shots of a scene at say -1,0 and +1 then open up the middle image in Camera Raw (assuming you are shooting raw and with a Canon). Now take the exposure slider and move it one stop over and one stop under. Did either the shadows or highlights blow out when you did that ? If not, then you have all of the dynamic range detail that you need in the one shot, you don't need the -1 or the +1. You just need to use the Raw tools to bring out the detail where you need it. That's something to bear in mind as anything that reduces the amount of work you need to do on an image is a big bonus.

So, back to HDR. Now that you have your set of bracketed images, load them up into your choice of software and use the presets and sliders in a way that gets all of the required detail correctly exposed but without turning into a cartoon. Using the sliders correctly will allow you to get all of the required detail out of the highlights and shadows. In the landscape example you should be able to get a good balance between your foreground (land) and background (sky) as you would if you were using a grad filter.

I hope this has been of use to you. If not, well you managed to get this far so something must have kept your attention :-)